Security printers face a constant challenge in staying ahead of counterfeiters, who have increasing access to advanced replication technology. For example, printed elements and diffractive relief structures, which are commonly used as security features in banknotes, can be susceptible to reproduction by optical scanning and contact copying, respectively.
The security document designer must therefore employ increasingly technically advanced features for securing documents against forgery. At the same time, it is desirable for these security features to be easily recognisable as such, so that a person scrutinising the document is aware of what to look for in an authentic document.
One commonly employed and easily recognisable feature is a printed image, for example in the form of a portrait of a famous person. Traditionally, such images were applied to banknotes by techniques such as intaglio printing, often including a guilloché (fine line) pattern. The result was an image having a characteristic appearance and tactile impression which, at the time, was difficult for counterfeiters to reproduce.
More secure authentication features can be produced by employing optically variable structures, such as diffraction gratings, which change in appearance as the observer changes their angle of view relative to the structure.
The above features can be combined by providing a security device having a tonal image which also exhibits an optically variable diffractive effect. Such an image can be produced by dividing a digitised (bitmapped) version of an input optically invariable image into an array of N×M pixels, as described in WO 91/03747. If the original image is black and white, then each pixel will have a greyscale value (grey level) corresponding to the brightness of the corresponding part of the image. A diffraction grating element can then be assigned to each pixel, the curvature of the grooves in each grating element being dependent on (e.g., inversely proportional to) the grey level of the corresponding pixel. The net effect produced to a person viewing the security feature is a halftone or continuous tone monochrome image which changes tone and/or colour as the viewing angle changes.
The security feature described above, known as a “Pixelgram”, is thus a mapping from an input continuous-tone image to a mosaic of diffraction gratings having both an optically variable component (tone/colour change on change of viewing angle) and an optically invariable component (continuous-tone portrait), thereby providing enhanced resistance to counterfeiting over a security feature having either component on its own.
In view of the continuing need to increase the difficulty to the counterfeiter, it is desirable to provide an improved security device having additional levels of authentication to those offered by known security devices such as those described above.